Mineral oil lubricants are derived from various crude oil stocks by a variety of refining processes which are directed towards obtaining a lubricant base stock of suitable boiling point, viscosity, viscosity index (VI) and other characteristics. Generally, the base stock will be produced from the crude oil by distillation of the crude in atmospheric and vacuum distillation towers, followed by the separation of undesirable aromatic components and finally, by dewaxing and various finishing steps. Because aromatic components lead to high viscosity, poor viscosity indices and poor oxidative stability, the use of asphaltic type crudes is not preferred as the yield of acceptable lube stocks will be extremely low after the large quantities of aromatic components contained in such crudes have been separated out; paraffinic crude stocks will therefore be preferred but aromatic separation procedures will still be necessary in order to remove undesirable aromatic components. In the case of the lubricant distillate fractions, generally referred to as the neutrals, e.g. heavy neutral, light neutral, etc., the aromatics will be extracted by solvent extraction using a solvent such as phenol, furfural or N-methylpyrrolidone (NMP) or another material which is selective for the extraction of the aromatic components. If the lube stock is a residual lube stock, the asphaltenes will first be removed in a propane deasphalting step followed by solvent extraction of residual aromatics to produce a lube generally referred to as bright stock. In either case, however, a dewaxing step is normally necessary in order for the lubricant to have a satisfactorily low pour point and cloud point, so that it will not solidify or precipitate the less soluble paraffinic components under the influence of low temperatures.
A number of dewaxing processes are known in the petroleum refining industry and of these, solvent dewaxing with solvents such as methylethylketone (MEK) and liquid propane, has been the one which has achieved the widest use in the industry. Recently, however, proposals have been made for using catalytic dewaxing processes for the production of lubricating oil stocks and these processes possess a number of advantages over the conventional solvent dewaxing procedures. The catalytic dewaxing processes which have been proposed are generally similar to those which have been proposed for dewaxing the middle distillate fractions such as heating oils, jet fuels and kerosenes, of which a number have been disclosed in the literature, for example, in Oil and Gas Journal, Jan. 6, 1975, pp. 69-73 and U.S. Pat. Nos. RE 28,398, 3,956,102 and 4,100,056. Generally, these processes operate by selectively cracking the longer chain end paraffins to produce lower molecular weight products which may then be removed by distillation from the higher boiling lube stock. The catalysts which have been proposed for this purpose have usually been zeolites which have a pore size which admits the straight chain, waxy n-paraffins either alone or with only slightly branched chain paraffins but which exclude more highly branched materials and cycloaliphatics. Zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35 and ZSM-38 have been proposed for this purpose in dewaxing processes, as described in U.S. Pat. Nos. 3,894,938, 4,176,050, 4,181,598, 4,222,855, 4,229,282 and 4,247,388. A dewaxing process employing synthetic offretite is described in U.S. Pat. No. 4,259,174.
Although the catalytic dewaxing processes are commercially attractive because they do not produce quantities of solid paraffin wax which presently is regarded as an undesirable, low value product, they do have certain disadvantages and because of this, certain proposals have been made for combining the catalytic dewaxing processes with other processes in order to produce lube stocks of satisfactory properties. For example, U.S. Pat. No. 4,181,598 discloses a method for producing a high quality lube base stock by subjecting a waxy fraction to solvent refining, followed by catalytic dewaxing over ZSM-5 with subsequent hydrotreatment of the product. U.S. Pat. No. 4,428,819 discloses a process for improving the quality of catalytically dewaxed lube stocks by subjecting the catalytically dewaxed oil to a hydroisomerization process which removes residual quantities of petrolatum wax which contribute to poor performance in the Overnight Cloud Point test (ASTM D2500-66). This process is intended to overcome one disadvantage of the intermediate pore dewaxing catalysts such as ZSM-5 which is that the normal paraffins are cracked much faster than the slightly branched chain paraffins and cycloparaffins so that, although a satisfactory pour point is attained (because the straight chain paraffins are removed) residual quantities of branched chain paraffins and cycloparaffins may be left in the oil, to contribute to a poor performance in the Overnight Cloud Point test when the oil is subjected to a relatively low temperature for an extended period of time. During this time, the petrolatum wax which is made up of the less soluble slightly branched chain paraffins and cycloparaffins, nucleates and grows into wax crystals of a sufficient size to produce a perceptible haze. Although it would be possible to remove the petrolatum wax by operating the dewaxing process at a higher conversion so that these components were removed together with the straight chain paraffins, the yield loss which would result, has generally been considered unacceptable
As mentioned above, the conventional catalytic dewaxing processes using intermediate pore size zeolites such as ZSM-5 operate by selectively cracking the waxy components of the feed. This results in a loss in yield since the components which are in the desired boiling range undergo a bulk conversion to lower boiling fractions which, although they may be useful in other products, must be removed from the lube stock. A notable advance in the processing of lube stocks is described in U.S. Pat. Nos. 4,419,220 and 4,518,485, in which the waxy components of the feed, comprising straight chain and slightly branched chain paraffins, are removed by isomerization over a catalyst based on zeolite beta.
During the isomerization, the waxy components are converted to relatively less waxy isoparaffins and at the same time, the slightly branched chain paraffins undergo isomerization to more highly branched aliphatics. A measure of cracking does take place during the operation so that not only is the pour point reduced by reason of the isomerization but, in addition, the heavy ends undergo some cracking or hydrocracking to form liquid range materials which contribute to a low viscosity product. The degree of cracking is, however, limited so as to maintain as much of the feedstock as possible in the desired boiling range. As mentioned above, this process uses a catalyst which is based on zeolite beta, together with a suitable hydrogenation-dehydrogenation component which is typically a base metal or a noble metal, usually of group VIA or VIIIA of the Periodic Table of the Elements (the periodic table used in this specification is the table approved by IUPAC), such as cobalt, molybdenum, nickel, tungsten, palladium or platinum. As described in U.S. Pat. No. 4,518,485, the isomerization dewaxing step may be preceded by a hydrotreating step in order to remove heteroatom-containing impurities, which may be separated in an interstage separation process similar to that employed in two-stage hydrotreating-hydrocracking processes.
The zeolite beta dewaxing process has significant advantages for dewaxing extremely waxy feeds, for example, Pacific and South-East Asian gas oils which may have upwards of 50 percent paraffins. Enhanced utilization of the properties of zeolite beta may, however, be secured by utilizing it in combination with other processing steps. For example, European Patent Application Publication No. 225,053 (corresponding to U.S. application Ser. No. 793,937, filed Nov. 1, 1985, now U.S. Pat. No. 5,179,103 and U.S. Pat. No. 4,919,788) utilizes an initial hydroisomerization step using a zeolite beta catalyst followed by a selective dewaxing over ZSM-5 or ZSM-23 or even solvent dewaxing to produce a product of high V.I. and low pour point. The initial hydroisomerization effectively removes waxy components from the back end of the feeds by isomerizing them to high V.I. isoparaffins and the subsequent selective dewaxing step preferentially removes front end waxes to obtain the target pour point. Extremely waxy stocks such as slack wax and deoiled wax are of particular utility in this process, as described in U.S. Pat. No. 4,975,177. Conventional high pressure hydroisomerization processes used in the production of very high V.I. lubes (120-145 V.I.) typically employ pressures over 1500 psig (about 10,440 kPa). See, for example, Developments in Lubrication PD19(2), 221-228 (Bull). Unlike these, the zeolite beta isomerization process operates well at low to moderate hydrogen pressures e.g. 300-1250 psig (about 2170-8720 kPa) and is therefore readily accommodated in existing low pressure refinery units e.g. Catalytic Hydrodesulfurization (CHD) units. In addition, the feed for the zeolite beta isomerization process may be obtained from various refinery streams including slack waxes and deoiled waxes as mentioned above as well as straight run gas oil (VGO) and deasphalted oil (DAO). The conventional high pressure process, however, usually employs wax feeds of specific character derived from aromatics extraction or hydrocracking of a crude prior to dewaxing.
Regardless of the nature of the feed, certain problems may arise. One is that a certain degree of cracking takes place during the isomerization process at the acidic sites on the zeolite beta catalyst. This cracking will cause dealkylation of some of the long chain alkyl substituted aromatic components so that cracking products including polycyclic aromatics within the lube boiling range but of extremely poor V.I. and oxidation stability are obtained. These components may adversely affect the properties of the final lube product. In addition, a disparity between the pour point (ASTM D-97 or equivalent method e.g. Autopour) and cloud point (ASTM D-2500-66) may develop, as described above as a result of certain waxes, primarily of a naphthenic character remaining in the oil after the isomerization-dewaxing step.
European Patent Application 0 464 547 discloses the preparation of high VI lubricants by treating a waxy feed which is hydrocracked to remove aromatic components followed by hydroisomerization over a low acidity zeolite beta catalyst, e.g. Pt/zeolite beta which contains boron as a framework component to give a low alpha value. The removal of aromatics in the first step permits use of lower hydroisomerization temperatures in the second step which limits 650.degree. F.+conversion and enhances paraffin isomerization selectivity. However, it is necessary to further treat the hydroisomerized product by dewaxing, e.g., solvent dewaxing with methylethylketone.